Akka Remote - Messages are not delivering (Java)

Trying to use Akka for parallel computing but facing a problem while communicating actors. I have two different actors (whose names are sender and receiver) which works on two different systems working on different ports on same ip address (whose names are SenderSystem and Receiver System). What I want to do is send message from sender actor to receiver actor. But on console I see a message like this

[INFO] [08/15/2015 12:36:51.645] [SenderSystem-akka.actor.default-dispatcher-4] [akka://SenderSystem/sender] Message [com.aliyesilkanat.akkadeneme.Messages$1] from Actor[akka://SenderSystem/deadLetters] to Actor[akka://SenderSystem/sender] was not delivered. [1] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.